Talisker

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* prometheus: add optinal support for promethues_client
* celery: request id automatically sent and logged, and support for 4.0
* docs: initial 'talisker contract'
* statsd: better client initialisation
* internal: refactoring of global variables, better /_status/ url dispatch

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* remove use of future's import hooks, as they mess with raven's vendored imports
* slight tweak to logfmt serialisation, and update docs to match

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*Upgrading*

This release includes a couple of minor backwards incompatible changes:

1) access logs now use the talisker format, rather than CLF. See the docs for
more info. If you are using access logs already, then the easiest upgrade
path is to output the access logs to stderr (access_logfile="-"), and delete
your old log files.

2) talisker no longer prefixes developer supplied tags with 'svc.'. This should
only matter if you've already set up dashboards or similar with the old
prefixed name, and you will need to remove the prefix

Changes:

* access logs now `in logfmt
<http://talisker.readthedocs.io/en/latest/logging.html#gunicorn-logs>`_
rather than CLF

* dummy statsd client is now useful `in testing
<http://talisker.readthedocs.io/en/latest/statsd.html#testing>`_

* logs are colored in development, to aid reading

* the 'svc' prefix for tags has been removed

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* actually include the encoding fix for check endpoint

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* add celery metrics
* fix issue with encoding in check endpoint when iterable

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* make celery runner actually work, wrt logging